1. Field of the Invention
The present invention relates to a discharge lamp apparatus, which drives a high voltage discharge lamp and is preferably used as a vehicle front light.
2. Description of Related Art
Various discharge lamp apparatuses are proposed (e.g., JP-A-9-180888 (U.S. Pat. No. 5,751,121) and JP-A-8-321389), which use a high voltage discharge lamp (lamp) as a vehicle front light, drives the lamp by alternating current (a.c.) voltage after boosting a voltage of a vehicle-mounted battery by a transformer and switching the polarity of the high voltage by an inverter circuit.
This lamp is mounted inside of a reflector provided at a vehicle front part. When an electric wiring part of the lamp is grounded accidentally, an excessive current flows and melts a fusible link or damage circuit devices in the discharge lamp apparatus.
Further, a switching device is provided at a primary side of a voltage boosting transformer to control a primary current, and controls electric power supplied to the lamp by pulse width modulation (PWM) control based on a lamp voltage and a lamp current. In this PWM control, when the duty ratio is increased to increase the electric power of the lamp, the secondary side output of the transformer decreases oppositely. Therefore, a maximum duty ratio is set to limit the duty ratio to be less than a maximum.
However, if the maximum duty ratio is set as above, the lamp can not be supplied with sufficient electric power when the lamp does not continue to light because of decrease in the lamp current at the time of starting lighting the lamp.
Still further, in the above discharge lamp apparatus, an electronic unit for the lamp is encased within a ballast housing, and the ballast housing is mounted outside of the lamp. Thus, extra space is required at the outside of the lamp for installing the electronic unit.